(1) Field of the Invention
The present invention refers to an electromagnetic radiation absorber based on magnetic microwires.
The invention is encompassed within the technical field of magnetic materials, also covering aspects of electromagnetism, applicable in the field of magnetic sensors and absorbers and the field of metallurgy.
(2) Description of Related Art
Numerous applications require eliminating reflections from electromagnetic radiation. The large number of electronic systems built into vehicles gives rise to an increase in electromagnetic interferences. This problem includes false images, radar interferences and a decrease in performance due to the coupling between various systems. A microwave absorber might be very effective for eliminating this type of problems. There is even greater interest in reducing the radar cross section of certain systems to prevent or minimize their detection.
Microwave absorbers are carried out by modifying the dielectric properties, or in other words the dielectric permittivity, and magnetic properties, or magnetic permeability, of certain materials. The first case involves dielectric absorbers basing their operation on the quarter wavelength resonance principle. However, the second case involves the absorption of the magnetic component of radiation. The first attempts made to eliminate reflections include Salisbury's screen absorber method, the non-resonant absorber, the resonant absorber, and resonant magnetic ferrite absorbers. In the case of Salisbury's screen (U.S. Pat. No. 2,599,944), a screen with a carefully chosen electrical resistance is placed at the point where the electrical field of the wave is maximum, i.e. at a space equal to a quarter wavelength with respect to the surface which is to be shielded. This method has little practical use since the absorber is too thick and is effective only for excessively narrow frequency bands and variations of angles of incidence.
In non-resonant methods, the radiation traverses a dielectric sheet to be subsequently reflected by the metallic surface. The dielectric sheet is thick enough so that, in the course of its reflection, the wave is sufficiently attenuated before reemerging from the sheet. Since the sheet must be made of a material having low losses at high frequencies and low reflection properties to assure penetration and reflection, the sheet must be very thick so as to effectively attenuate the wave.
In the first resonant methods, materials with high dielectric losses are placed directly on the conductive surface that is to be protected. The dielectric material has an effective thickness, measured inside the material, that is about equal to an even number of quarters of half-wavelengths of the incident radiation. The usefulness of the method is limited due to the large thickness of the dielectric sheet and the narrow absorption band they have, particularly at low frequencies. Attempts have been made to eliminate these deficiencies by dispersing ferromagnetic conductive particles in the dielectric. However, when metallic particles are dispersed, high permeabilities in the order of 10 or 100 are not compatible with low conductivities in the order of 10−2 or 10−8 mohm per meter.
Another type of absorbers are those known as ferrite absorbers (U.S. Pat. No. 3,938,152), which have clear advantages in comparison with those already described herein. They function in the form of thin sheets such that they overcome the disadvantages of the large thickness required by dielectric absorbers. They are furthermore effective for frequencies between 10 MHz and 15,000 MHz and dissipate more energy than dielectrics.
The ferrite absorbers developed up until now eliminate reflections by means of insulating or semiconductive ferrite sheets, and particularly ferrimagnetic metal oxides, placed directly on the reflecting surfaces. In these cases the term ferrite refers to ferrimagnetic metal oxides including, but not limited to, compounds such as spinel, garnet, magnetoplumbite and perovskites.
In this type, absorption is of two types, which may or may not occur simultaneously. They are dielectric and magnetic losses. The first losses are due to electron transfer between the cations Fe2+ and Fe3+, while the losses of the second type originate from the movement and relaxation of magnetic domain spins.
According to certain inventions (U.S. Pat. No. 3,938,152), at low frequencies, generally those in the range between UHF and the L-band, energy is predominantly extracted from the magnetic component of the incident radiation field, whereas at higher frequencies, generally in the L-band and higher, energy is extracted equally from the electric and magnetic components.
This type of absorbers eliminates reflection because the radiation establishes a maximum magnetic field on the conductor surface. In the normal incidence of a planar wave on an ideal conductor total reflection occurs, the reflected intensity being equal to the incident intensity. The incident and reflected waves then come together, generating a standing wave in which the electrical field is nil at the conductor boundary, whereas the magnetic field at this boundary is maximum. There is magnetic field condensation during the maximum possible time. It is therefore necessary, in the case of ferrite, for the incident radiation to traverse the absorbent sheet so as to establish the maximum magnetic field conditions. It has been seen that the complex part of the permeability of certain ferrimagnetic metal oxides varies with frequency, such that it allows obtaining low reflections over very broad frequency ranges without needing to use magnetic absorbers with high thicknesses as in other cases.
Taking into account the reflection coefficient in metals for normal incidence, it is deduced that when working with a thin sheet the reflected wave can be attenuated independently of the electric permittivity of the absorbent material. Minimum reflections will occur at a given frequency if the complex permeability μ″ is substantially greater than the real permeability μ′, provided the product Kτ<<1, where K is the wave number and τ is the thickness of the sheet.
Taylor's technique for manufacturing microwires, which allows obtaining microwires with very small diameters comprised between one and several tens of a micron through a simple process, is known. The microwires thus obtained can be made from a wide variety of alloys and magnetic and non-magnetic metals. This technique is described, for example, in the article “The Preparation, Properties and Applications of Some Glass Coated Metal Filaments Prepared by the Taylor-Wire Process” W. Donald et al., Journal of Material Science, 31, 1996, pp 1139-1148.
The most important characteristic of the Taylor process is that it allows obtaining metals and alloys in microwire form with an insulating sleeve in a single and simple operation, with the cost-effectiveness that this implies in the manufacturing process.
The technique for obtaining magnetic microwires with an insulating sleeve and amorphous microstructure is described, for example, in the article “Magnetic Properties of Amorphous Fe—P Alloys Containing Ga, Ge and As” H. Wiesner and J. Schneider, Stat. Sol. (a) 26, 71 (1974), Phys. Stat. Sol. (a) 26, 71 (1974).
The properties of the magnetic amorphous microwire with an insulating sleeve related to the object of the present invention are described in the article “Natural ferromagnetic resonance in cast microwires covered by glass insulation” A. N. Antonenko, S. A. Baranov, V. S. Larin and A. V. Torkunov, Journal of Materials Science and Engineering A (1997) 248-250.
The alloys used for manufacturing the microwire core are of the transition metal-metalloid type, and have an amorphous microstructure. The effect of the microwire geometry on its magnetic performance is due to the magnetoelastic character of the alloys used, which in turn depends on the magnetostriction constant thereof.